1. Field of the Invention
The present invention relates to a magneto-optical information reproducing apparatus which utilizes the magneto-optical effect to reproduce information magnetically recorded on a recording medium.
2. Related Background Art
In recent years, optical memories for effecting recording and reproduction by a laser beam have been actively studied and developed for practical use as high-density recording memories. Of these, magneto-optical disks capable of erasing and re-writing have been regarded as promising, with optical disks exclusively for reproduction typified by compact disks and direct read after write (DRAW) type optical disks. Magneto-optical disks are such that information is magnetically recorded by the utilization of the localized temperature rise of a magnetic thin film caused by the application of a laser spot thereto and the information is reproduced by the magneto-optical effect (particularly the Kerr effect.) The Kerr effect refers to the phenomenon that the plane of polarization is rotated when light is reflected by a magnetic recording medium.
The basic construction of a magneto-optical disk apparatus according to the prior art is shown in FIG. 1 of the accompanying drawings. In FIG. 1, the reference numeral 1 designates a semiconductor laser, the reference numeral 2 denotes a collimator lens, the reference numerals 11 and 20 designate half-mirrors, the reference numeral 4 denotes an objective lens, the reference numeral 6 designates a magneto-optical recording medium, the reference numerals 7.sub.1 and 7.sub.2 denote analyzers, the reference numeral 8 designates a condensing lens, and the reference numerals 9.sub.1 and 9.sub.2 denote photodetectors. The direction of P-polarization is parallel to the plane of the drawing sheet, and the direction of S-polarization is perpendicular to the plane of the drawing sheet.
Description will now be made of a case where magneto-optical information is reproduced in such apparatus. A light beam emitted from the semiconductor laser 1 as a rectilinearly polarized light in the direction of P-polarization is collimated by the collimater lens 2 and passes through the half-mirror 11. If the P-polarized component amplitude transmittance is t.sub.p and the S-polarized component amplitude transmittance is t.sub.s, .vertline.t.sub.p .vertline..sup.2 =.vertline.t.sub.s .vertline..sup.2 =0.5 in the half-mirror 11. The light beam is imaged as a minute spot on the magneto-optical recording medium 6 by the objective lens 4. When a magnetic section (pit) is pre-formed on the medium 6, as shown in FIG. 2 of the accompanying drawings, the reflected light from the medium 6 is subjected to the rotation of the plane of polarization of .+-..theta..sub.k by the Kerr effect in conformity with whether the direction of magnetization of the spot-applied area is upward or downward. Here, if the P-polarized component of the amplitude reflectance of the recording medium 6 is R and the S-polarized component is K, the following equation is established: ##EQU2## The magneto-optically modulated reflected light is again collimated by the objective lens 4 and reflected by the half-mirror 11, whereafter it is made into a convergent light beam by the condensing lens 8 and divided by the half-mirror 20, and the divided lights pass through the analysers 7.sub.1 and 7.sub.2, respectively, and are detected as intensity-modulated light beams by the photodetectors 9.sub.1 and 9.sub.2. That is, as shown in FIG. 2, the angle of the optic axis of the analyzer with respect to the direction of P-polarization is .+-..theta..sub.A on the transmission side and the reflection side, and the light beam is analyzed as a regular projection of the amplitude thereof onto the optic axis of the analyzer.
Considering that the Kerr rotation angle is of the order of 1.degree. and that the magneto-optical modulated component is of a very minute amount, it is necessary that the azimuth angle .theta..sub.A of the optic axis of the analyzer be set to such an optimum position that the C/N (the ratio between the carrier wave and the noise) of the detection signal becomes maximum. For example, in U.S. Pat. No. 4,569,035 issued on Feb. 4, 1986, there is shown an example of an apparatus using as a photodetector an avalanche photodiode (APD) or the like having a multiplying action wherein the azimuth of the transmission axis (the optic axis) of the analyzer is optimized. On the other hand, in an apparatus using a photodetector a PIN photodiode or the like having no multiplying action, the azimuth angle .theta..sub.A of the optic axis of the analyzer has been set to 45.degree. so that the signal light becomes maximum. When the Kerr rotation angle is +.theta..sub.k, if the quantity of light incident on the recording medium is I.sub.O, the quantities of light passing through the analyzers on the transmission side and the reflection side and entering the respective photodetectors are: ##EQU3## Since .theta..sub.k .about.1.degree., .vertline.R.vertline..sup.2 &gt;&gt;.vertline.K.vertline..sup.2 is established and thus, equation (2) can be expressed as follows: ##EQU4## In equation (3), the second term in the parentheses is the magneto-optical modulated component and the first term in the parentheses is the non-modulated component. These lights are photoelectrically converted by the photodetectors 9.sub.1 and 9.sub.2, respectively, and differentially detected by a differential circuit, not shown, whereby a magneto-optical signal is obtained. However, when the noise superposed on the detection signal is taken into consideration, maximum C/N cannot always be obtained by providing .theta..sub.A =45.degree..
On the other hand, a magneto-optical information reproducing apparatus using a polarizing beam splitter instead of the half-mirror 11 shown in FIG. 1 to improve the C/N of the above-mentioned reproducing signal is proposed in U.S. Pat. No. 4,561,032 issued on Dec. 24, 1985. Further, an example in which the polarizing characteristic of this polarizing beam splitter is set so that C/N is maximum is disclosed in U.S. Pat. No. 4,558,440 issued on Dec. 10, 1985. However, in these examples as well, the azimuth angle of the optic axis of the analyzer is defined as 45.degree., and no mention is made of optimizing this azimuth angle and a specific method therefor.